Where the state of stenosis has occurred in a vessel of a living body, such as a blood vessel, or in particular an artery, the technique of percutaneous angioplasty (PTA) is routinely applied. This is the surgical procedure of introducing a balloon mounted to the vicinity of the distal end of a catheter to the stenosed lesion, with the balloon then being expanded to hold open the stenosed lesion to secure the blood flow.
It is known that, even with the application of PTA, there is a high risk of restenosis occurring in the stenosed lesion.
In order to prevent restenosis, a stent is implanted into the vessel to which PTA has been applied. The stent, implanted into the blood vessel in the constricted state, is subsequently dilated and implanted into the blood vessel to support the vessel wall from inside to prevent restenosis. Such stents formed of metal, e.g., stainless steel, Ti—Ni alloy, or of a resin material or of biodegradable polymer have already been proposed and are already known to be in use.
A stent delivery system is used for delivering the stent to a desired site in the vessel. The stent delivery system has a different stent holding mechanism, depending on the method used for expanding the stent.
For example, a stent delivery system exists in which a balloon expandable stent, contracted by plastic deformation, is expanded by a balloon and kept in the expanded state even after removal of the balloon. Therefore the stent may be implanted into a desired site in the vessel.
This stent delivery system comprises a balloon in a contracted state at the distal end of the catheter, and a mechanism for loading and holding a balloon-mounted stent, with the stent being in a contracted state under the effect of plastic deformation. The catheter carrying the balloon at its distal end, as used here, is termed a balloon catheter.
For implantation of the stent into a desired site, the stent delivery system comprising the balloon catheter and carrying the stent, is introduced into the vessel.
When the stent has been delivered to the desired site of implantation, a fluid material, such as a contrast medium, which is an X-ray impermeable liquid material, is supplied to the balloon via the catheter to dilate the balloon. Such expansion of the balloon dilates the stent mounted to the outer periphery of the balloon.
Another stent delivery system, a self-expandable stent, contracted under an external pressure and expanded on removal of the external pressure, to a desired site of implantation in the vessel, is provided with a mechanism for holding the stent by a protective sheath for preventing self-expandable of the stent. This protective sheath is adapted for covering the outer periphery of the stent, mounted in a contracted state to the distal end of the catheter inserted into a vessel of the living body. This stent delivery system holds the stent in the contracted state by means of the protective sheath and delivers the stent to the target site in the vessel without the stent incidentally becoming detached from the catheter. When the stent mounted to the distal end of the catheter has been delivered to the target site in the vessel, the protective sheath is moved relatively to the catheter, whereby the stent is expanded from the contracted state, so as to be self-expanded and left at the desired site in the vessel.
Meanwhile, the stent delivery system for the balloon expandable stent is of such a structure that the catheter carrying the stent exposed to the outer surface of the balloon is introduced into the vessel. Therefore, while the stent is being delivered to the site of implantation in the vessel, the stent tends to cause some injury to the inner wall of the vessel, as it comes into contact with the vessel wall directly. In particular, if the stent is delivered in the exposed state from the catheter or balloon to a target site in the blood vessel, which is of small diameter and is bent or meandering, the blood vessel wall coming into contact with the stent may readily be injured. Moreover, if the stent is made of metal, sharp or rigid sites may be produced on the surface of the stent. If the above-described stent is introduced into the vessel in the exposed state, the inner wall of the vessel may be easily injured. Additionally, in the stent delivery system for the balloon expandable stent, there are cases of the stent becoming detached from the stent delivery system during its delivery to the target site. This occurs when the balloon expandable stent is mounted directly on the balloon without the protective sheath.
In the stent delivery system for the self-expandable stent, the protective sheath is mounted on the outer periphery of the stent, which is loaded at the distal end of the catheter. Thus, the stent portion of the delivery system for the self-expandable stent is larger in outer diameter than the stent portion of the delivery system for the balloon expandable stent.
It is difficult to deliver the stent to a target site with a large outer diameter the stent delivery system and especially so if the desired target site has a small inner diameter. Furthermore it is impossible to implant the stent if the inner diameter of the desired target site is smaller than the outer diameter of the stent delivery system. Consequently, the self-expandable stent has limitations in terms of target site, as compared to the balloon expandable stent.
In the case whereby the stent delivery system for the self-expandable stent delivers the stent in a tortuous blood vessel, the protective sheath and the stent should be delivered together in the bent or meandering blood vessel. Therefore, it is difficult to remove the protective sheath smoothly from the stent and to implant the stent into the target site. Moreover it is extremely difficult to release the stent, loaded on the distal end of the catheter, from the contracted state brought about by the protective sheath, and to implant the stent thus released at the desired site in the vessel.
In order to introduce the stent into the vessel which is not only of small diameter and tortuous, but also is consolidation, the characteristics of the stent delivery system are needed to pass the stenosed site easily and with sufficient tractability, otherwise the stent will be unable to reach the desired target site.
Consequently, the shape and mechanical properties of the stent delivery system, which deliver the various types of stent to the desired target site, require a small outer diameter, flexibility and surface smoothness.